Ca signals in bovine adrenal chromaffin cells were studied both in Fura-2/AM-loaded intact cells, and in voltage-clamped cells under whole-ceU patch-clamp conditions. The effects of gamma-aminobutyric acid b subtype (GABAb) receptor activation on K+-depolarization-induced signals and on voltage-activated Ca 2+ currents were investigated. Both GABA (20~tM) plus bicucuUine (20 ~tM) and (-)baclofen (20-100 ~tM), effectively inhibited the Ca signal in intact cells. The effects caused by baclofen continued to develop during the time interval between two successive stimuli. The restoration of the Ca signal during washout of baclofen was also delayed and continued in some experiments for 10-20 rain. The inhibitory effect of baclofen on the Ca signal was eliminated by pre-treatment of the cells with pertussis toxin (PTX, l~tg/ml, for 4-6h at 37~ Baclofen (50 ~tM) inhibited Ca 2+ current in whole-cell mode by at most 20%. The effect developed quickly and was reversible. Infusion into the cells of a non-hydrolyzable analogue of guanosine 5'-triphosphate GTPTS (I00 ~tM), led to complete inhibition of the Ca z+ conductance and of voltage-evoked intracellular Ca ([CA]i) transients within 2min. In paired cells intracellularly perfused with GTPyS-free solution, the Ca 2+ current amplitude decreased by only about 30~ for 5-6 min. It is concluded that bovine chromaffin cells have functional GABAb receptors the activation of which, mediated by a PTX-sensitive GTP-binding protein, inhibits the evoked increase in 2+ 2+ cytosolic free Ca . The small size of the effect on Ca current in whole-cell mode as compared to that on the Ca signal in intact cells may be explained by washout of some regulatory element during cell dialysis, or by a relatively small contribution of the normal voltage-activated Ca a+ current to the Ca signal. Alternatively, it might indicate GABA b effects on mechanisms other than Ca 2+ channels.